Such a device is known and described, for example, in DE-PS No. 29 25 027 corresponding to U.S. Pat. No. 4,280,322.
Such devices are used, for example, in the case of two-for-one twisting spindles in which the thread sensor is formed by the thread guide eyelet which limits the upper end of the thread balloon and which, with the thread running, is held by this in its operating position and is fastened to a holder which can be swivelled in the vertical direction. As a result of the centrifugal and air resistance force acting on the thread in the case of two-for-one twisting spindles, the thread is given a tensile force in the balloon region. Accordingly, thread forces are active at the balloon which can be utilised for switching functions. Upon a thread breakage the thread sensor moves out of the operating position into a switching position in which the switching member is actuated, whereby various procedures, e.g. registration, switching off and the like are triggered at the working location. Then by hand or by a servicing device the switching element can be moved out of its rest position into a braking position in which it exerts a control function in such a way that under specific conditions a re-starting of the spindle is prevented. Now the necessary maintenance procedures can be carried out. After restoration of the switching element, the spindle is to start up again. So that, through the thread sensor which is still in the switching position, on account of balloon force which is not yet fully developed in the start-up phase, the monitoring mechanism is not immediately switched on again and thus the switch-off procedure is triggered afresh, in the case of the known device upon the restoring of the switching element a delay member becomes effective, which bridges or respectively prevents the function, triggering a stoppage of the spindle, of the thread sensor until the full thread force is built up and the thread sensor is disposed in the operating position.
The known device has the disadvantage that the pneumatic control mechanism of the monitoring mechanism is constructed in a relatively complex manner with two control valves between which a compressed-air store is connected, in which respect one of the two control valves is connected mechanically to the switching element. In this respect at least one of the two control valves has to be arranged in each case in the vicinity of the switching element, so that a direct mechanical coupling is not possible without excessively great expenditure. This has the result that the control valves in the case of machines having several twisting spindles are distributed over wide regions of the machine and cannot be combined and be arranged at constructionally particularly favourable locations.